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Re: st: How to get marginal effect of quadratic term in tobit model in stata 11.0


From   lutunnahar begum <pikitardif@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: How to get marginal effect of quadratic term in tobit model in stata 11.0
Date   Wed, 20 Jul 2011 08:19:44 +1000

Thank you very much for your help, Maarten.

Best,

Lutfunnahar

On Tue, Jul 19, 2011 at 6:58 PM, Maarten Buis <maartenlbuis@gmail.com> wrote:
> On Tue, Jul 19, 2011 at 1:05 AM, lutunnahar begum wrote:
>> I tried the following command:
>>
>> gen tb2=tb^2
>> tobit ch tb tb2 .........., ll(0) robust
>> margins, dydx(*) predict (ystar (0,.)) post
>>
>> I also tried -dtobit- with -version 8.2- and got the 4 MEs  for squared term.
>>
>> I understand that the true ME should not consider the squared effect
>> separately as there is only one variable. But papers using tobit model
>> report the marginal effect of squared term. I am confused whether I
>> should use the above mentioned command to get this.
>
> Remember that squared terms are just interaction terms of a variable
> with itself. In non-linear models the "marginal effect" of the product
> term is meaningless, see Norton, Wang, and Ai (2004). So, the authors
> in those papers that included the marginal effects of the interaction
> terms made a mistake. There are ways of getting "correct" marginal
> effects, but those are not worth the effort as they are likely to
> confuse more than help. If you added a quadratic term than your
> variable will have a non-linear effect, and the way to show that is
> with graphs.
>
> Hope this helps,
> Maarten
>
> Edward Norton, Hua Wang, and Chunrong Ai (2004) "Computing interaction
> effects and standard errors in logit and probit models" The Stata
> Journal 4(2):154--167.
> <http://www.stata-journal.com/article.html?article=st0063>
>
> --------------------------
> Maarten L. Buis
> Institut fuer Soziologie
> Universitaet Tuebingen
> Wilhelmstrasse 36
> 72074 Tuebingen
> Germany
>
>
> http://www.maartenbuis.nl
> --------------------------
>
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